A fluid fuel is understood hereinbelow to refer especially to fuel oil and/or fuel gas, as used especially for gas turbines. Fuel oil is understood to refer to all combustible liquids, e.g. mineral oil, methanol, etc. and fuel gas is understood to refer to all combustible gases, e.g. natural gas, coal gas, synthesis gas, biogas, propane, butane, etc. Such burners involving a catalytic reaction are disclosed for example in document EP-A-491 481.
Such burner systems are also suitable for applications in turbomachines such as, for example, gas turbines. A gas turbine normally consists of a compressor part, a burner part and a turbine part. The compressor part and the turbine part are normally located on a common shaft which simultaneously drives a generator for generating electricity. In the compressor part, pre-heated fresh air is compressed to the pressure required in the burner part. In the burner part, the compressed and pre-heated fresh air is burned with a fuel such as e.g. natural gas or fuel oil. The hot burner exhaust gas is fed to the turbine part and pressure is released there such that work is performed.
When the compressed and preheated fresh air is burned with the fuel gas, pollutants, for example nitrogen oxides NOx or carbon monoxide CO, emerge as particularly undesirable combustion products. The nitrogen oxides are deemed along with sulfur dioxide to be a principal causal agent of the environmental problem of acid rain. There is therefore the determination—also on account of strict legal thresholds specified for NOx emission—to keep the NOx emission of a gas turbine especially low and at the same time not to affect the performance of the gas turbine to any great extent.
Thus, for example, reducing the flame temperature or the peak flame temperature in the burner part has the effect of reducing the nitrogen oxides. To do this, steam is fed into the fuel gas or the compressed and preheated fresh air or water is sprayed into the combustion chamber. Such measures which reduce per se a nitrogen oxide emission of the gas turbine, ate referred to as primary measures for reducing nitrogen oxides. Correspondingly, all measures in which nitrogen oxides contained at one time in the waste gas of a gas turbine—or of a combustion process in general—are reduced by means of subsequent measures are referred to as secondary measures.
The method of selective catalytic reduction (SCR), in which the nitrogen oxides together with a reducing agent, preferably ammonia, are bonded to a catalyst, thereby forming harmless nitrogen and water, has come to be used worldwide for this purpose. The use of this technology however, necessarily involves the consumption of reducing agents. The catalytic converters for nitrogen oxide reduction disposed in the exhaust-gas duct cause by their nature a fall in pressure in the exhaust-gas duct which brings with it a decline in output of the turbine. Even a decline in output of the order of a few parts per thousand has a severe impact, where the gas turbine has an output of, for example, 150 MV and an electricity selling price of approximately 8 cents per kWh of electricity, on the profit achievable with such a plant.
Recent thoughts on burner design tend toward replacing a customary diffusion burner normally used in the gas turbine or a swirl-stabilized premix burner with a catalytic combustion system. With a catalytic combustion system, lower nitrogen oxide emissions are achieved simply by virtue of the combustion process as such than is possible with the conventional types of burner mentioned above. The known disadvantages of the SCR method (large volumes of catalysts, consumption of reducing means, marked loss of pressure) can in this way be overcome.
One application of a catalytic process is disclosed in EP 0 832 397 B1, for example, which shows a catalytic gas turbine burner. Here, a part of the fuel gas is drawn off by means of a conduit system, routed via a catalytic stage and then fed into the fuel gas again in order to reduce its catalytic ignition temperature. The catalytic stage is fashioned here as a preforming stage which comprises a catalytic converter installation which is provided for converting a hydrocarbon contained in the fuel gas into an alcohol and/or an aldehyde or H2 and CO.
EP 0 832 399 B1 discloses a burner for burning a fuel in which the fuel outlet of a catalytic auxiliary burner to stabilize the main burner with the catalytic combustion of a pilot fuel flow is provided upstream of the fuel outlet of the main burner in the direction of flow of the fuel within a flow channel. In this case, the catalytic auxiliary burner is disposed centrally and the main burner coronally relative to the cross-section of the flow channel for the fuel.
The catalytic combustion systems described hereinabove consist here of a catalytic converter which is disposed axially. Only a part of the energy contained in the fuel is released in the catalytic converter, as a result of which stabilization of the burnout of the remaining part of the chemically bound energy is improved in a combustion space in an axial direction downstream of the catalytic converter. This primary reaction commences after a certain period, known as the autoignition time, which depends essentially on the temperature and the composition of the gas at the catalytic converter outlet.
The use of such known arrangements for operation with markedly different fuels is usually a problem in this context, since the catalytic converter generally has to be specifically adapted for certain fuels. In particular, this also makes it difficult to use a catalytic converter which has been designed for natural gas as a reactor for converting long-chain hydrocarbons (in particular, therefore pre-evaporated fuel oil) since the corresponding reaction kinetic properties are significantly different. Such arrangements are therefore only of limited suitability for enabling operation of the gas turbine with a liquid fuel.